The present invention relates in general to space diversity radio receivers with spaced antennas, and more specifically, to a diversity radio receiver system with proportional mixing of output signals from different tuners attached to different spaced antennas.
Space diversity radio receiver systems are a well known attempt to reduce the effects of multipath distortion in mobile receivers. Multipath distortion is a localized effect resulting from interaction between signals from a transmitter which traverse different paths to reach a receiving antenna. By switching between spaced antennas in a diversity radio receiver, specific multipath events can be avoided since the spacing of the antennas helps insure that not both of the antennas will experience the same multipath event at the same time.
In short time delay multipath, reflected radio signals from nearby objects combine in anti-phase with the direct radio signal at certain locations resulting in the partial or total cancellation of the signal. An undesirable burst of noise known as a "spit" or "pop" is heard as the signal to noise ratio of the received signal decreases. When a vehicle antenna passes through these multipath points, the signal strength voltage detected in the radio tuner decreases momentarily due to the decreased signal strength at the antenna. Proportional diversity systems are known, such as described in patent application GB-2257605, wherein audio signals from separate tuners connected to separate diversity antennas are combined in a manner proportional to the signal strength measured at each tuner. Since the signal strength voltage drops for a particular tuner during a multipath event, its relative contribution to the combined signal is likewise decreased. This provides an improvement in the audio signal during short time delay multipath events by attenuating the spits which may be present at one antenna, but not at the other.
Long time delay multipath is a type of multipath distortion which cannot be accurately detected by monitoring the signal strength voltage and which therefore has gone uncorrected by prior art systems. In longtime delay multipath, reflected radio signals from more distant objects (e.g., 500 meters or more from the receiving antenna) combine with the direct radio signal. Due to the longer time delay between the signals, the signals are not correlated and therefore do not cause a cancellation of the received signal but rather the superposition or overlap of the signals' frequency spectra. This spectral overlap causes audio distortion which is not accompanied by a drop in the signal strength (in fact, the signal strength may increase), and, as a result, prior proportional diversity systems do not provide an improvement under these conditions.